fephisto: try this: bring as many defenders plus 4 fast offence troops (Knights, Horsemen - to kill wounded enemis) plus 1 settler.
Found a city (preferable land your troops on a hill screened by a river), rsuh barrakcs, walls. Now, the enemy will try to take that city, but he should fail!
This is better than an ordinary beachhead, because your units can heal, get more defence bonus, and you can reinforce via air once you have airports!
Bring in bombard units as well and fortify them in the city, they'll take HP off attacks, too.
Now, bring in the offensive arm and go conquer!